It is known to prepare aldehydes and alcohols by reacting olefins with carbon monoxide and hydrogen. The reaction is catalyzed by hydridometal carbonyls, preferably those of metals of group VIII of the Periodic Table. In addition to cobalt, which is used extensively in industry as a catalyst metal, rhodium has recently become increasingly important. In contrast to cobalt, rhodium permits the reaction to be carried out at low pressure; also, straight-chain n-aldehydes are formed preferentially, and iso-aldehydes only to a minor extent. Finally, the hydrogenation of olefins to form saturated hydrocarbons occurs to a considerably lesser extent when rhodium catalysts are used than when cobalt catalysts are used.
In the processes which have been introduced into industry, the rhodium catalyst is employed in the form of modified hydridorhodium carbonyls which additionally and optionally contain excess ligands. Ligands which have proven particularly successful are tertiary phosphines or phosphites. Use of these compounds enables the reaction pressure to be reduced to values below 30 MPa.
However, the separation of the reaction products and recovery of the catalyst dissolved homogeneously in the reaction product cause problems. In general, separation is effected by removing the reaction product from the reaction mixture by distillation. In practice, however, this method can only be used in the hydroformylation of lower olefins having up to about 8 carbon atoms in the molecule, due to the thermal sensitivity of the aldehydes and alcohols formed. In addition, it has become apparent that heating of the distillation material also leads to considerable losses in catalyst through decomposition of the rhodium complex compounds.
The problems described are avoided by using catalyst systems which are soluble in water. Such catalysts are described, for example, in German Patent 2,627,354. The solubility of the rhodium complex compounds is achieved there by using sulfonated triarylphosphines as the complex component. The removal of the catalyst from the reaction product, when the hydroformylation reaction is complete, is effected simply by separating the aqueous and organic phases, i.e. without distillation and thus without additional thermal steps. A further feature of this procedure is that n-aldehydes are formed with high selectivity from terminal olefins, and iso-aldehydes are formed only to a very minor extent. Besides sulfonated triarylphosphines, carboxylated triarylphosphines are also employed as the complex components of water-soluble rhodium complex compounds.
The known two-phase processes have proven highly successful in the hydroformylation of lower olefins, in particular ethylene and propylene. If higher olefins, such as octene or decene, are employed, the conversion and/or selectivity for n-compounds drops markedly. Hence, on an industrial scale, the reaction is frequently no longer economical.
Various methods have been used to overcome these difficulties. According to DE 3,412,335 A1, a solubilizer is added to the reaction medium. A disadvantage of this procedure is the use of reagents which are alien to the reaction, i.e. are not among the starting materials, the reaction products, or the catalytic substances. This means that a negative effect on the reaction proceedings and, in particular, on the life of the catalysts cannot be excluded.
The process described in DE 3,420,491 A1 for hyroformylation of olefins uses catalytically active rhodium complex compounds in which the complex ligands are quaternary ammonium salts of sulfonated triarylphosphines. The quaternary ammonium ions contain an alkyl or aralkyl radical having 7 to 18 carbon atoms and 3 straight or branched alkyls having 1 to 4 carbon atoms. This process has also proven very successful in the reaction of higher olefins. However, the high price of quaternary ammonium hydroxides, which are necessary for recovery of the quaternary ammonium salts of sulfonated triarylphosphines, stands in the way of its general use.